Explain EXPLAIN QUERY TREE (children 1)
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 3)
    ExpressionList (children 1)
     Function uniq (children 1)
      ExpressionList (children 1)
       Identifier t.a
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       Subquery (alias t) (children 1)
        SelectWithUnionQuery (children 1)
         ExpressionList (children 1)
          SelectQuery (children 3)
           ExpressionList (children 2)
            Identifier a
            Function sum (children 1)
             ExpressionList (children 1)
              Identifier b
           TablesInSelectQuery (children 1)
            TablesInSelectQueryElement (children 1)
             TableExpression (children 1)
              TableIdentifier test_rewrite_uniq_to_count
           ExpressionList (children 1)
            Identifier a
    Set
